TRUNCATE(7) PostgreSQL 9.2.7 Documentation TRUNCATE(7)NAMETRUNCATE - empty a table or set of tables
SYNOPSISTRUNCATE [ TABLE ] [ ONLY ] name [ * ] [, ... ]
[ RESTART IDENTITY | CONTINUE IDENTITY ] [ CASCADE | RESTRICT ]
DESCRIPTIONTRUNCATE quickly removes all rows from a set of tables. It has the same
effect as an unqualified DELETE on each table, but since it does not
actually scan the tables it is faster. Furthermore, it reclaims disk
space immediately, rather than requiring a subsequent VACUUM operation.
This is most useful on large tables.
PARAMETERS
name
The name (optionally schema-qualified) of a table to truncate. If
ONLY is specified before the table name, only that table is
truncated. If ONLY is not specified, the table and all its
descendant tables (if any) are truncated. Optionally, * can be
specified after the table name to explicitly indicate that
descendant tables are included.
RESTART IDENTITY
Automatically restart sequences owned by columns of the truncated
table(s).
CONTINUE IDENTITY
Do not change the values of sequences. This is the default.
CASCADE
Automatically truncate all tables that have foreign-key references
to any of the named tables, or to any tables added to the group due
to CASCADE.
RESTRICT
Refuse to truncate if any of the tables have foreign-key references
from tables that are not listed in the command. This is the
default.
NOTES
You must have the TRUNCATE privilege on a table to truncate it.
TRUNCATE acquires an ACCESS EXCLUSIVE lock on each table it operates
on, which blocks all other concurrent operations on the table. When
RESTART IDENTITY is specified, any sequences that are to be restarted
are likewise locked exclusively. If concurrent access to a table is
required, then the DELETE command should be used instead.
TRUNCATE cannot be used on a table that has foreign-key references from
other tables, unless all such tables are also truncated in the same
command. Checking validity in such cases would require table scans, and
the whole point is not to do one. The CASCADE option can be used to
automatically include all dependent tables — but be very careful when
using this option, or else you might lose data you did not intend to!
TRUNCATE will not fire any ON DELETE triggers that might exist for the
tables. But it will fire ON TRUNCATE triggers. If ON TRUNCATE triggers
are defined for any of the tables, then all BEFORE TRUNCATE triggers
are fired before any truncation happens, and all AFTER TRUNCATE
triggers are fired after the last truncation is performed and any
sequences are reset. The triggers will fire in the order that the
tables are to be processed (first those listed in the command, and then
any that were added due to cascading).
Warning
TRUNCATE is not MVCC-safe (see Chapter 13, Concurrency Control, in
the documentation for general information about MVCC). After
truncation, the table will appear empty to all concurrent
transactions, even if they are using a snapshot taken before the
truncation occurred. This will only be an issue for a transaction
that did not access the truncated table before the truncation
happened — any transaction that has done so would hold at least an
ACCESS SHARE lock, which would block TRUNCATE until that
transaction completes. So truncation will not cause any apparent
inconsistency in the table contents for successive queries on the
same table, but it could cause visible inconsistency between the
contents of the truncated table and other tables in the database.
TRUNCATE is transaction-safe with respect to the data in the tables:
the truncation will be safely rolled back if the surrounding
transaction does not commit.
When RESTART IDENTITY is specified, the implied ALTER SEQUENCE RESTART
operations are also done transactionally; that is, they will be rolled
back if the surrounding transaction does not commit. This is unlike the
normal behavior of ALTER SEQUENCE RESTART. Be aware that if any
additional sequence operations are done on the restarted sequences
before the transaction rolls back, the effects of these operations on
the sequences will be rolled back, but not their effects on currval();
that is, after the transaction currval() will continue to reflect the
last sequence value obtained inside the failed transaction, even though
the sequence itself may no longer be consistent with that. This is
similar to the usual behavior of currval() after a failed transaction.
EXAMPLES
Truncate the tables bigtable and fattable:
TRUNCATE bigtable, fattable;
The same, and also reset any associated sequence generators:
TRUNCATE bigtable, fattable RESTART IDENTITY;
Truncate the table othertable, and cascade to any tables that reference
othertable via foreign-key constraints:
TRUNCATE othertable CASCADE;
COMPATIBILITY
The SQL:2008 standard includes a TRUNCATE command with the syntax
TRUNCATE TABLE tablename. The clauses CONTINUE IDENTITY/RESTART
IDENTITY also appear in that standard, but have slightly different
though related meanings. Some of the concurrency behavior of this
command is left implementation-defined by the standard, so the above
notes should be considered and compared with other implementations if
necessary.
PostgreSQL 9.2.7 2014-02-17 TRUNCATE(7)
